There have been known aqueous inks containing various kinds of water soluble dyes. These aqueous inks have excellent solution stability, but have poor water-resistance in a written trade or printed image. However, aqueous inks having excellent water-resistance on and after writing down are very few.
In order to overcome the above defect in aqueous inks, various improvements have been proposed, for example, Japanese Pat. Publn. (unexamined) Nos. 31526/1976, 115306/1978, 116762/1981, 131678/1981, 10685/1982, 30772/1982, 102972/1982, 198768/1982, 202359/1982, 1764/1983 and Japanese Pat. Publn. (examined) No. 6581/1965.
Among them, use of direct dyes was proposed. The direct dyes, however, do not provide sufficient water-resistance and do not have sufficient solubility in low pH range. In a high pH range, physical properties of ink are adversely affected for the reason that acetate fiber which is usually used as ink storage material is hydrolyzed to release the decomposed materials to the ink. Also the direct dyes do not have good color tone other than black, but the black has possibility of increasing the viscosity of the ink or to gel it.
An ink wherein an oil-soluble dye is dissolved in an aromatic solvent provides sufficient water resistance, but it blots on a paper and smells because of the organic solvent. Also an ink containing a spirit-soluble dye, such as an ink wherein an amine salt of the spirit-soluble dye is dissolved in an alcoholic solvent, has poor water-resistance in comparison with the ink using the oil-soluble dye.
Also an emulsion or dispersion ink containing an water-insoluble dye or water-insoluble pigment was proposed. It has, however, insufficient storage stability and may cause a plugging of a capillary passage of the storage material.
There exists an ink which makes a written trace water insoluble as time goes on in terms of a reaction of the ink with light or oxygen in the air. It requires however, a considerable time for obtaining sufficient water-resistance and may change its color.
Furthermore, it was also proposed that an ink be prepared by adding an aditive (which may include a polymer material) to the water-soluble dye, thus increasing the water-resistance of the ink. This ink is not expected to exhibit excellent water-resistance on or after writing down. This ink is not stable to a pH change so that the ink solution has to be kept in a narrow pH range. The additive ink having higher water resistance has worse solution stability.